In the prior art, turbomachines are known with contra-rotating propeller systems, the propellers of which are driven by a mechanical transmission device, typically assuming the form of a differential reducer. This differential reducer has a particular epicyclical gearing, the sun gear of which is made to rotate by a rotor of a power free turbine, whereof the planet carrier drives the first propeller, and the crown of which drives the second propeller. In this respect, it is noted that as a function of the position of the contra-rotating propellers relative to the power free turbine driving them, the first propeller constitutes the downstream propeller and the second propeller, the upstream propeller, or vice versa. Whatever the case may be, unlike a simple epicyclical gearing, the crown is not stationary, but mobile.
Usually, each of the first and second propellers comprises a hub centered on the longitudinal axis, an outer ferrule being arranged concentrically thereto and participating in outwardly radially delimiting a main annular tunnel of the turbomachine, as well as coupling arms connecting the outer ferrule to the hub.
Moreover, a casing is provided, inserted between the power free turbine and the first and second propellers. This casing has a casing extension towards the closest propeller, this extension rotatably supporting the hub of said propeller.
This configuration means that a significant part of the aforementioned propeller, or even the entirety thereof, is axially offset from the casing extension ensuring its rotatable support. This results in a cantilever that is delicate to manage from a mechanical perspective, and generally requiring a substantial elongation of the casing extension, in particular with the aim of moving the rolling bearings apart so as to procure an acceptable rotational guiding.
This constraint amounts to an axial elongation of the propeller system, which is costly both in terms of overall mass and bulk.